Air compressors of the rocking rod piston type, also termed wobble piston type, when they involve overrun intake ports, tend to stall. This is particularly the case with small capacity induction type A.C. motors of low starting torque. By the embodiment of the present invention, the peaks of starting torque are reduced at the expense of a small loss in efficiency, but in general in this type of equipment utilizing the present invention the certainty of starting outweighs the relatively minor reduction in efficiency.